1. Field of the Invention
This invention relates generally to solid state processing of high softening temperature materials (HSTM) through friction stirring (FS), including friction stir processing (FSP), friction stir mixing (FSM), friction stir welding (FSW), and friction stir spot welding (FSSW). The invention relates more specifically to selecting the geometry of the friction stirring tool that is used to perform all of the friction processes named above. This document will refer to specific processes, but all of the above named friction processes should be considered to be within the scope of whatever process is specifically being discussed.
2. Description of Related Art
The prior art teaches numerous tool geometries and configurations for friction stir welding of low softening temperature materials (LSTM) such as aluminum alloys. These tool geometries are exclusively for LSTM applications because tool materials and geometries had not yet been discovered to join or process high softening temperature materials (HSTM) such as steel, stainless steel, and nickel base alloys.
While designed for HSTM applications, it should be stated that some of the tool geometries disclosed in this document are functional for friction stirring of aluminum alloys (i.e. scrolls on concave and convex shoulders, threads on pins, etc.). However, some of the geometries are not technically feasible because the strength of the tool material is exceeded during solid state processing. These geometries include fins that resemble air foils that extend radially from the tip of the pin.
A careful review of FSW tool geometries used in current art for LSTM is based on tool geometries that have been successful through trial and error attempts using a variety of FSW parameters (i.e. tool rotational speed, Z axis loading, and tool travel speed). Subsequent publications and patent applications have disclosed variations of these tool geometries, some of which have not been tested and are not feasible for these LSTMs.
There are also ongoing programs established to develop computer models of the FSW process. The purpose of these computer models is to predict and develop tool geometries that can provide consolidated friction stir welds with favorable microstructures at optimum tool travel and rotational speeds. To this point in time, millions of dollars have been spent and there has not been a successful model that can predict tool geometries for FSW of LSTM. FSW and friction stir processing (FSP) of HSTM has been performed at Brigham Young University. Limited modeling of FSW and FSP has been completed under DARPA research contracts. Successful computer models have not yet been developed because of the complexity of the FSW process and extensive validation testing that must be correlated with actual FSW tool geometries and FSW/FSP specimens.
These experiments have demonstrated that there are two predominant problems with the existing art. First, tool geometries are developed using a trial and error approach. Second, tool geometries for LSTM are very often different than tool geometries required for HSTM. Even though some geometries appear similar for both HSTM and LSTM, there are key differences that must be accounted for in the HSTM design.
The trial and error approach which has been adopted for FSW and FSP of LSTM has prevented an understanding of process fundamentals and tool design principles. This approach has been tolerable since the cost of tool fabrication is minimal and limited to the cost of tool steel, machining, and heat treatment. A tool design can be readily made and tested to determine if the desired microstructure and mechanical properties have been achieved. Since costs are minimal, most tool geometries are developed in this fashion. This approach is based on observation and luck and does not explain the mechanism of FSW as it relates to LSTM. The scientific and engineering fundamentals of tool design have been largely overlooked and are only partially understood.
A second problem that is not understood by those attempting tool design is that tool geometries for HSTM are generally unsuitable for use in LSTM. For example, a threaded pin design is an acceptable pin design for LSTM such as aluminum alloys. When this same tool design is made to friction stir weld steel, the threads from the pin remain engaged in the steel and the pin will break from the tool as the tool is extracted.
Because most LSTM consist of aluminum alloys, tool geometries reflect the trial and error design approach that has been successful in these alloys. However, there are at least three important fundamental differences in the design of tools for FSW/FSP for HSTM and LSTM. First, the thermal conductivity of the most common LSTM materials (aluminum and copper) is greater than that of the tool (typically tool steel).
In contrast, the thermal conductivity of the HSTM materials is generally significantly lower than that of the tools used in the welding. A second fundamental difference is in the coefficient of friction between the tool and the workpiece. For LSTM, the coefficient of friction is high, while it is low for HSTM. This factor significantly changes the heat generation of the process, which has important considerations for tool design. The third difference is in the primary objective of a successful process. In most LSTM, the objective is to minimize the heat input to preserve the optimal microstructure, which leads to high operating forces.
In contrast, the primary objective in HSTM is to soften the material sufficiently to achieve full consolidation, which is a significant obstacle to successful friction stirring of HSTM. These three differences are generally going to require fundamental differences in tool design for most HSTM.
It is noted for the purposes of this document that HSTM should be considered to include materials such as metal matrix composites, ferrous alloys such as steel and stainless steel, and non-ferrous materials and superalloys. Superalloys can be materials having a higher melting temperature than bronze or aluminum, and may have other elements mixed in as well. Some examples of superalloys are nickel, iron-nickel, and cobalt-based alloys generally used at temperatures above 1000 degrees F. Additional elements commonly found in superalloys include, but are not limited to, chromium, molybdenum, tungsten, aluminum, titanium, niobium, tantalum, and rhenium. Titanium should also be considered to be within the class of materials being considered. Titanium is a non-ferrous material, but has a higher melting point than other nonferrous materials.
It should also be understood that solid state processing is defined herein as a temporary transformation into a plasticized state that typically does not include a liquid phase. However, it is noted that some embodiments allow one or more elements to pass through a liquid phase, and still obtain the benefits of the present invention. Solid state processing should also be considered as a term that describes the plasticization of the HSTM during friction stirring (FS), friction stir processing (FSP), friction stir mixing (FSM), friction stir welding (FSW), and friction stir spot welding (FSSW).
The tool being used for solid state processing as defined herein can be assumed to be comprised of a shank or shaft, a shoulder on the shank, and a pin disposed on the shoulder. In an alternative embodiment, the tool has no shoulder, but is a conical pin having any of the features to be described herein.